The intensity–velocity relationship of the Čerenkov light emitted by μ-mesons passing through Plexiglas has been studied for the velocity range 0.83c to 0.99970c (84 Mev. to 4.2 Bev.). No departure of the observed from the theoretical intensity was found in the high velocity region, in disagreement with the results of Bassi, Bianchi, and Manduchi (1952). In the low velocity region slightly less radiation was observed than expected, but this is attributed to an optical effect in the apparatus rather than to a departure from theory of the Čerenkov light emission.